Hitherto, there is disclosed an inner-wadding type writing implement wherein an ink occlusion body is impregnated with a reversible thermal discoloration aqueous ink composition capable of forming handwriting whose color tone changes depending on a change in temperature and is housed in a barrel (see, e.g., Patent Document 1).
The above reversible thermal discoloration aqueous ink composition solves a problem that a reversible thermal discoloration microcapsule pigment is gradually flocculated and precipitated in the ink occlusion body owing to a difference in specific gravity between the reversible thermal discoloration microcapsule pigment and a vehicle and hence handwriting is lightened or deepened in color depending on the state where a pen body is downward (an inverted state) or upward (an erecting state), and solves the problem by incorporating a water-soluble polymer flocculant into the ink to make the microcapsule pigment suspended in a loose flocculated state through a loose crosslinking action of the polymer flocculant.
However, it is difficult to completely suppress the lightening and deepening in color of handwriting attributable to the difference in specific gravity between the microcapsule pigment and the vehicle merely by incorporating the polymer flocculant. Particularly, it is desired to develop a means for preventing handwriting from lightening with time in an erecting state or from lightening induced by the application of vibration at transportation or at the time when it is carried on in an erecting state.    [Patent Document 1] JP-A-11-335613